I'°" .
NON·CIRCU
Auburn University Libraries
llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
~ CQJ~T ~ ~ emt?
File D 13.413 / Spark Plug/ 42
(HEAVIER-THAN-AIR)
PUBLISHED BY THE CHIEF OF AIR SERVICE, WASHINGTON, D. C.
Vol. I September 15, 1920 No. 20
EFFECT OF REDUCED ENGINE COMPRESSION
PRESSURE ON PRE-IGNITION WITH
MICA PLUGS
V
Prepared by Engineering Division, Air Service
McCook Field, July 22. 1920
WASHINGTON
GOVERNMENT PRINTING OFFICE
1920
. Ralph Brov~ • . -, " · · ·
LIBRAh,
MAR 2 6 2013
Non-Oepoitorv
Auburn Univ-ersi·tv
EFFECT OF REDUCED ENGINE COMPRESSION PRESSURE
ON PRE-IGNITION WITH MICA PLUGS.
OBJECT OF TEST.
The object of this test was to determine the highest
maximum engine compression pressure which can be used
on tests of representative mica spark plugs in the Liberty
single-cylinder engine without obtaining preignition.
CONCLUSIONS.
It is possible to run Brewster-Goldsmith and K. L. G.
mica spark plugs in the Liberty single-cylinder engine
without obtaining preignition, provided the compression
pressure at normal speed does not exceed 86 and 106
pounds per square inch, respectively, for the two plugs.
INTRODUCTION.
Heretofore mica spark plugs tested in the single-cylinder
Liberty engine at full throttle have shown a marked
tendency to preignition, although the same makes of
plugs have given satisfactory service in flight tests. The
two principal conditions in which ,the single-cylinder test
differs from flight service are that there is less air circulation
around the plug and the cylinder pressures are higher,
since all running in the single-cylinder engine is done at
ground level. Both of these factors cause the plug to
run hotter than in flight service. This test was conducted
in order to devise a method to offset the higher temperature
conditions, and to make possible the preliminary
t esting of mica spark plugs on the single-cylinder Liberty
engine.
METHOD OF TEST.
Two B. G. and two K. L. G. mica plugs were tested in a
single-cylinder Liberty engine. The throttle travel on
the control sector was divided into 10 equal divisions.
One pair of the plugs to be tested was inserted in the cylinder
and the engine run for five minutes at 1,700 r.p.m.
with the throttle open three-tenths of its movement.
Any tendency toward preignition was noted. At the end
of five minutes, the ignition was shut off and any tendency
toward preignition was noted. The throttle opening was
then increased one-tenth and the run repeated. The
throttle opening was increased by tenths progressively
and the runs repeated until preignition was obtained.
The throttle was then decreased one division, and a halfhour
run made, the switches being tried every five minut es,
and the engine torque watched for signs of preignition. At
the end of the half-hour run the throttle opening was
again 1I1creased one division and a five-minute run made,
any tendency toward preignition being noted.
RESULTS OF TEST.
The results of the test are shown in the appended tables.
No engine trouble was experienced during the test and
there were no signs of preignition until a throttle opening
was reached at which pronounced preignition occurred.
9402-20
ANALYSIS OF RESULT
It was found that there was a definite throttle opening
and corresponding compression pressure, at which satisfactory
operation of the plugs could be obtained and
above which preignition occurred . Both makes of plug
did not preignite at the same compression pressures. The
critical compression pressure for tJ:ie Brewster-Goldsmith
and K. L. G. plugs appears to be 86 and 106 pounds per
square inch, respectively, at 1,700 r. p. m. In actual
service these pressures are not often exceeded, since most
flying is done either with the throttle pa1tially closed, or,
if at full throttle, at altitudes at which the atmospheric
pressures are sufficiently low to bring the compression
pressures below the critical point._ On the basis of the
difference in atmospheric pressurn the altitude at which
the compression pressure on the Liberty engine would
fall below the critical point is probably well below 5,000
feet. This probably explains why preignition is not
perceptibly encountered in flight service with these plugs.
(3)
0
Throttle Revoluopening
tions per
(ten ths). minute.
Five minute runs.
B. G. Mica Plugs.
Horsepower.
Preignition
during
run.
Preigni- Com-tion
with pression
ignition with
offat end OkilJ
of run. indicator.
,---- ,------- ------- --
3
4
5
6
5
5
5
5
5
5
6
3
4
5
6
7
8
7
7
7
7
7
7
8
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
12.0 None . ... None . •.... . .. . ....
16. 0 . . . do . ...... . do ..... . . .. . . .. . .
21. o .. . do ...... .. do. .... 86
24. 5 Present . Present 1 96
21.0 None . ... None. ... 86
21. O • • • do . ....... do... . . 86
21. O •• • do ... .. ... do..... 86
21. O • •• do . .... . .. do.. ... 86
21. O • •• do ... : . . . . do.. ... 86
21. 0 ... do ........ do.. ... 86
24. 5 ... do... . . Present. 96
1 Within 30 seconds.
K. L. G. Mica Plugs.
1,700
1, 700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
1,700
10.0 None .... None .•.. ....•.•...
13. O • •• do ...... . . do . .... •. . .. ... ..
22. O ••• do ........ do ... •... . ...•. •.
26. o •.• do . ... . ... do . . . . . .. . . . . . . •.
27. 8 . .. do .. ..... . do... . . 106
29. 9 . .. do..... Present. 112
28.0 • . . do ...•. None .. .. 106
27. 8 ... do .... • . . . do. . ... 106
27.8 . . . do .• . .. . .. do..... 106
27. 8 . . . do .. . . .... do..... 106
27. 8 •.. do .... . . . . do..... 106
27. 8 . .. do ........ do... . . 106
30. O Present . Present . 112
•
Air Service Information Circulars
